Polymers offer unique avenues for the structural control of materials on the nanoscopic length scale for the production of nanoporous media, membranes, lithographic templates, and scaffolds for assemblies of electronic materials. [1±4] With structures on this length scale, quantum properties of electronic materials are exhibited even at elevated temperatures. The natural length scale of polymer chains and their morphologies in the bulk lie precisely at these length scales and, as such, there is a substantial effort to produce, characterize and use polymeric nanostructures. The ease of processing polymers adds to the attractiveness of polymer-based nanostructures. In comparison to the time-intensive process of sequential writing of nanoscale patterns, nanostructure formation by self-assembly is highly parallel and inherently fast. Block copolymers are ideal materials in this respect, since, due to the connectivity of two chemically distinct chains, the molecules self-assemble into ordered morphologies with a size scale limited to molecular dimensions. Of particular interest are block copolymers that form cylindrical microdomains, since the elimination of the minor component transforms the material into an array of nanopores.A prerequisite for the use of copolymers is the control over the orientation of the microdomains. In particular, for cylindrical microdomains, an orientation normal to the substrate surface is desirable. Two different approaches are used to this end. In thin films, random copolymers anchored to a substrate can be used to produce a neutral surface. [5] For entropic reasons, the microdomains orient normal to the substrate surface. [6] In a second approach, electric fields were used to orient the cylindrical microdomains parallel to the field lines. [7±10] The approach relies on the orientation-dependent polarization energy induced when an anisotropic body is placed in an electric field. An anisotropic microphase structure will orient such that the interfaces between the two blocks are aligned parallel to the electric field.In this article it is shown that cylindrical microdomains of a copolymer film can be used to generate an array of ordered nanoscopic pores with well-controlled size, orientation, and structure. To this end, selective etching procedures and a characterization of the samples by quantitative analysis of the X-ray scattering along with electron (EM) and atomic force microscopies (AFM) are described. The processes outlined are shown to be operative over a very large range in sample thickness ranging from 40 nm up to several micrometers. The resulting nanoporous films are promising candidates as membranes with specific transport properties and as templates for electronic and magnetic nanostructured materials. Figures 1A and 1B show AFM images obtained from a 40 nm±thick film prepared on a neutral substrate after annealing. Cylinders standing perpendicular to the substrate are clearly discernable, particularly in the phase image, since the height variations are very small. Polystyr...
Unexpected morphological changes have been found in a number of aliphatic polyesters (poly(ecapro1actone) and the suberate, azelate, and sebacate polyesters of ethylene glycol) upon blending with small concentrations ( N 1 %) of compatible polar polymeric diluents, notably poly(viny1 butyral) and poly(viny1 chloride). Among these changes are striking reduction in frequency of nucleation of spherulites, marked enhancement in regularity of lamellar organization in banded spherulites (also manifested by improved regularity of banding), modification of interlamellar spacings, and, in some instances, modification of molecular packing in crystals. It is believed that adsorption of diluent on crystal boundaries (growth faces and fold surfaces of lamellae) underlies most of these effects. Preliminary experiments indicate that similar morphological changes may occur in nylon 66 and nylon 610 when blended with poly(vinylpyrro1idone). IntroductionThere have been numerous investigations of spherulitic crystallization from the melt in compatible blends in which an uncrystallizable polymeric diluent has been added to a crystallizable host polymer. Early work involved blends of isotactic and atactic isomers of the same polymer and in recent years has been extended to blends of chemically dissimilar polymers. In general, incidence of nucleation of spherulites and their subsequent rate of growth are both suppressed by incorporation of diluent. Seldom, however, are they suppressed by as much as an order of magnitude unless the diluent is present in substantial concentration (125%) and most blends previously studied have involved diluent concentrations of at least 10%. Associated changes in morphology usually appear unremarkable in that they reflect either segregation of relatively large amounts of diluent between lamellar crystals or changes in texture that, from their similarity to changes of texture brought about in undiluted host polymer by varying molecular weight, have been ascribed to modification of fluidity in the melt. Our purpose here is to report new observations of striking and more subtle morphological changes induced in aliphatic polyesters and in nylons by blending with certain diluents in small concentration (-1% ). Incidence of nucleation is also acutely sensitive to the presence of these diluents whereas growth rates of spherulites are not.The experiments outlined below derive originally from observations that addition of poly(viny1 butyral) (PVB), poly(viny1 formal) (PVF), or poly(viny1 chloride) (PVC) to poly(ecapro1actone) (PCL) has the effect of inducing "banding" (occurrence of ringed extinction patterns) in spherulites of this host polymer in a range of crystallization temperatures in which it does not normally occur. Such a change upon addition of a diluent is not of itself without precedent in polymer morphology* and in blends involving PCL in particular2+ but, in this instance, it was attended by several unusual and unprecedented features. Among these were (a) substantial reduction in frequency of nuclea...
Thin films of block copolymers have been used as templates and scaffolds for the fabrication of arrays of nanostructured materials. In general, a chemical modification of the film or the removal of one of the components by photodegradative methods is required to produce a nanoporous film that serves as a template or scaffold. Here, however, the preferential interaction of one of the components with a solvent is shown to produce a reconstruction of the block copolymer film that, upon drying, leads to the generation of a nanoporous template. The area density of the pores is identical to that of the original copolymer thin film. Since no chemical reactions occurr, the process is fully reversible. Upon heating the copolymer film above its glass‐transition temperature, mobility is imparted to the copolymer and the original copolymer film with oriented domains is recovered. The film reconstruction significantly simplifies the generation of nanoporous templates.
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